Are IPS hulls shallower V than shaftdrive?

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Oops, should be a reply to Nautical.

[/ QUOTE ]Understood. But I'm with him on that.
It really depends on how big those big seas are.
When at times you have green water sweeping the f/b, you don't want to be there regardless of visibility. I've been cruising in conditions where I've forbidden anybody onboard to go up there, full stop.
 
Thankfully I was going with the weather instead of head to, did some fantastic imitations of a surf board though ! did a 39 from Douglas to Liverpool early morning in the dark head on though (another crap Irish sea forecast) that was interesting! down to 6 knots at one point near knocked my fillings out, could not tell where the formation was coming from, wind 90 degrees over tide and no shape nor make just seemed to bash us every direction bit like being on the spin cycle :-) . Not recommended at all, and too my embarrassment made the wrong call, should have turned back after the first hour but with pressure of transport arriving that afternoon, lifts booked, return flights booked decided to push on, shouldn't have, bad seamanship on my part could have all gone pear shaped very quickly. Sounds all great, horrah ! I am hero , nope !, was scary and past the capability and design envelope of the boat albeit a good one. Nil points to Nautical !.
 
Yes see your point there Nick, my experience though is when it gets really lumpy and the wind is really zinging especially off the bow quarter the amount water and spray flying at you makes it really hard work, you get tired and cold quickly and start to loose concentration.
 
Yeah Trev, sometimes at sea sh!t happens, and recognizing our own bad seamanship is part of the seamanship improvement process, I guess.
I've also been a bit bold in my judgement, in that occasion I mentioned in my previous post.
And being confident in the boat capabilities is no excuse, the sea can always win when it wants to, also against battleships.
 
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IPS was adopted by the sports cruiser builders long before many of the flybridge builders

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Actually, AFAIK, the first IPS boat introduced to the UK was the Rodman 41 FB which appeared at LIBS a few years back but, yes, I agree that it seems to have been used more on SC boats than FB so far. Maybe that's because it's harder to get right on FB boats or maybe it's because builders believe SC buyers will value the increased efficiency/handling more. Basically FB buyers are a more conservative bunch
Interesting that Abs modify hull design too to incorporate IPS and I accept what you say regarding sensitivity to trim. Maybe also this sensitivity makes IPS on FB harder to get right. Is Abs looking at Zeus or is this a state secret!
 
Agree with you, Nick H. A FB is far better in rough seas from the visibility point of view as, not only have you got 360deg viz but also you are much higher. I also believe that the inside helm positions of most FB's are better than the helm position of most SC's, particularly canvas canopy SC's, which often have relatively low windscreens so that you have to look partially thru the plastic canopy windows when driving and the canopy leaks. Hardtop SC's are usually better
 
Absolutely right Mike, Rodman were one of the Co's that got involved early on, Azimut did too but held back from production, the only one they put into production later was the 43S although they have been tinkering with the bigger engines recently. On the whole though the whole IPS thing has been driven by the Sports Cruiser builders but there are quite a few FB chaps now experimenting with IPS and Zeus, I guess trying to make up ground lost to the fully enclosed HT / pod drive SC's, no doubt in certain size ranges they have lost ground to the later.

We are looking at everything at the moment, Zeus has some advantages over IPS not least cost, if it stacks up, performs better, and gives the customer a better all round package then it is on the cards. The biggest downside at the moment is the availability of service and dealer network, VP are everywhere and spannermen are pretty well clued up on IPS now which is all part of the aftersales support to customers.
 
I understand your concerns re servicing. If Cummins marine service is anything like the service they offer for the construction equipment market, you'll have some very pissed off customers /forums/images/graemlins/smile.gif
 
If the water, rather than the spray, is coming over the flybridge (and i've had it once as well), then lots more water would be landing in an open sportcruiser in the same conditions. If the sport cruiser is a hard top, then that's no better or worse than the lower helm of a flybridge. So, either way, I don't see how the vision from a sportcruiser can be better.
 
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As a matter of fact, there is an inherent flaw in using pod drives on any V planing hulls, not just flybridges: when steering, they generate rotating forces on the hull longitudinal axis, and the deeper the V, the higher the angle and the stronger those forces. There's nothing they can do to avoid that, aside from flattening the hull (which obviously has other disadvantages). They could have made the pods much longer, in order to allow them rotate horizontally instead of at the same angle of the hull, but surely there were other disadvantages, additional drag and draft being the most obvious that spring to mind.

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Mapis, I dont get your logic/engineering here. sterndrive boats, shafts with rudders, as well as IPS, create a twisting force about the boat's fore-aft longitudinal axis when steering. It's not unique to IPS. Also, if you mount an IPS on a "wedge", so it's steering axis is pure vertical, you still get this twisting force. Or am i misunderstanding you?
 
For my 2 pen'orth - I've driven the Antares 12 (which started this thread) in both Flyer 12 HT coupe form with IPS, and the FB version on shafts. I have to say that the hull is a goodun, and the 2 version both work well. In the coupe you get very sporty handling and good rough weather performance as well - the fine entry works well - but I can imagine that in certain conditions it could be a touch bow up, but vis is excellent. The FB on shafts was great - very quickly on the plane with no nose up intermediate stage at all, and a nice level ride that feels very secure. However, I don't think I'd like to be in the IPS version if it has the same trim settings on the drives as the Flyer 12 version cos in a hard turn you could spend quite a bit of time chatting to the fish over the side - not my idea of fun. Both very good boats though, just different beasts.
 
Interesting. I think one of the things that maybe has held IPS development back on FB boats is that the steering response is so much sharper than rudders that you get greater angles of heel so peeps on the flybridge feel less secure. I guess it's similar to an outdrive powered FB boat compared to a shaftdrive one.
 
Nope, you're not misunderstanding J, and in fact you're quite correct.
To completely eliminate the twisting moment when steering, it would be necessary to have:
1) propulsion/rudders aligned with the longitudinal axis crossing the CoG, and
2) evenly distributed water resistance against the hull, also when steered.
And neither are feasible on a surface vessel, just in a submarine.

But by varying the thrust angle also vertically, rather than just horizontally, such effect is increased.
I'm afraid I won't be able to explain that as technically as I once had the opportunity to hear from a naval architect, but for what I understood (and in my poor words) the problem is that IPS "pulls" the hull in the wrong direction: when steering let's say to port, the port pod is angled downward, thus pulling the port side of the hull further into the water, and the stbd pod is angled upward, pulling the stbd part of the hull further out.
Therefore, the normal twisting moment is amplified.
In fact, if it would make any sense to have the thrust rotating along the hull angle, why shouldn't sterndrives be installed perpendicular to the hull rather than perpendicular to the waterline? It's just the pod that has to deal with the movement restrictions introduced by the V hull, 'cause it's placed under it.
A sort of wedge would clearly eliminate at least this component of the twisting forces, but as I said they must have faced other disadvantages with such solution, though I must admit that this is just my guess.
 
Ah, thanks. I see your point, and agree.

I'd explain the maths differently though. If you take a 15 deg deadrise hull, and compare the twisting for a normal IPS fitted on an angle with the twisting force of rudders/outdrives/wedge-mounted IPS, the difference is a factor of Cosine 15 degrees. Ok, I'm taking a shortcut slightly with the geometry, but that's about right. That's an extra 4% torque only. In other words the IPS exerts a 4% greater twisting force, all other things being equal

Not a huge amount, but enoguh to make the boat lean a bit more.

At 20deg deadrise it becomes +6.5%
 
Don't be shy, you didn't explain the math differently.
You explained the math, whilst I didn't at all! /forums/images/graemlins/smile.gif
Otoh, I'm pretty sure that - according to that engineer - the effect was more relevant, to the point of being potentially dangerous.
I can't remember exactly, but he mentioned also some relation with:
a) speed, and
b) time (duration) of steering
Something along the lines that, when steering at speed and keeping the boat steered for some time, the hull can keep increasing the leaning angle (unless throttles are pulled back a bit). Sorry, I can't remember any further detail, but that guy surely knew what he was talking about.
 
:-)

Yes, I can beleive that. My analysis was too simple really. As a boat rolls in a corner, its very axis of longitudinal rotation moves, and so you get a vicious circle effect. Hence, the overall effect is much more than just 4% or 6% etc.

Impossible to generalise, depnds on hull, but I can see the sense in what your guy said

Well, I've lost the bit of early enthusiasm I had for IPS. I've never had any desire at all for joystick parking, urgh, as I prefer to know what each prop is doing. And the fuel saving figures dont make it seem too good on larger boats anyway.
 
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I've never had any desire at all for joystick parking, urgh, as I prefer to know what each prop is doing.

[/ QUOTE ]100% agreed. And you know what, I wouldn't be surprised if the joystick thingie in the future would become a no no among the boating community.
I mean, that was clearly the main selling point of IPS so far, not the efficiency and so forth: I heard horror stories of dealers using the joystick as the focal wow factor with wannabe boaters, along the lines of "with this even your 10 yo child can handle the boat Sir, it's that easy. You can enjoy this fantastic 43' with no experience at all, don't worry" (with apologies to Trev, I'm sure he's NOT that kind of dealer).
But I'm also beginning to see boaters giggling at peeps who are considering the purchase of an IPS boat. As if it's starting to be seen as a sort of un-fashionable thingie for poorly skilled helmsmen (which, ermm... is what it is, actually - at least as far as the joystick is concerned).
Time will tell, but the economic situation could also work against IPS (paradoxically, in spite of its better efficiency) because it's much more attractive for newcomers than for experienced boaters, and these days I see less and less young high flyers with huge amounts of money burning holes in their pockets....
 
My guess us that the IPS drives, because they are steerable, exert a much greater turning force on the hull than inherently inefficient rudders so whilst the geometric difference may be 4% or 6% or whatever, the actual effect on the hull is greater
I'm trying to get my head around Mapism's point concerning the lean angle. One difference between IPS and rudders, as has been already pointed out, is that the IPS drives are mounted normal to the hull ie at an angle and rudders are mounted vertically. When the IPS drives are turned to steer the boat, one drive exerts an upward force on the hull and the other a downward force so that there is then a rotational force in the vertical plane on the hull which tends to increase the natural heel of the boat when turning. As long as the drives are turned, that rotational force is trying to heel the boat over and the only thing stopping the boat going right over is the buoyancy of the hull. As Mapism says, if the IPS drives were mounted vertically, this effect would be much reduced
For me, the jury is still out on IPS drives. I applaud Volvo for innovating something new that seems to have efficiency advantages over shaftdrive which is inherently very inefficient and with oil prices only going one way ie up in the future, that has to be a good thing. But, for me, reliability is as yet unproven in the long term and, as you say, many builders do not yet seem to have optimised the potential speed and mpg gains available from IPS. As for joystick control, I still get a quiet satisfaction from reversing my boat into a tricky crosswind Med mooring without denting anything just using the engines and no thruster so, IMHO, joystick control would make it too easy but then I'm probably a luddite in this respect /forums/images/graemlins/smile.gif
 
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As for joystick control, I still get a quiet satisfaction from reversing my boat into a tricky crosswind Med mooring without denting anything just using the engines and no thruster so, IMHO, joystick control would make it too easy but then I'm probably a luddite in this respect

[/ QUOTE ]Naah, you were just reading my mind while writing your post apparently.... /forums/images/graemlins/smile.gif
 
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